A variety of techniques are currently utilized in determining the presence and estimation of quantities of hydrocarbons (oil and gas) in earth formations. These methods are designed to determine formation parameters, including, among other things, the resistivity, porosity, and permeability of the rock formation surrounding the wellbore drilled for recovering the hydrocarbons. Typically, the tools designed to provide the desired information are used to log the wellbore. Much of the logging is done after the wellbores have been drilled. More recently, wellbores have been logged while drilling, which is referred to as measurement-while-drilling (MWD) or logging-while-drilling (LWD). One advantage of MWD techniques is that the information about the rock formation is available at an earlier time when the formation is not yet damaged by an invasion of the drilling mud. Thus, MWD logging may often deliver better formation evaluation (FE) data quality. In addition, having the formation evaluation (FE) data available already during drilling may enable the use of the FE data to influence decisions related to the ongoing drilling (such as geo-steering, for example). Yet another advantage is the time saving and, hence, cost saving if a separate wireline logging run can be avoided.
An important aspect of exploration is that of correlating geological formations across a reservoir. The dictionary definition of term “lithology”, which we adopt, is:                the character of a rock formation or of the rock found in a geological area or stratum expressed in terms of its structure, mineral composition, color, and texture.Methods have been developed for producing images of the borehole wall using electrical, acoustic, gamma ray and density measurements using sensors conveyed into the borehole on a bottomhole assembly. See, for example, U.S. Pat. No. 6,594,837 to Kurkoski; U.S. Pat. No. 7,256,582 to Fulda et al., U.S. Pat. No. 5,502,686 to Dory et al., all having the same assignee as the present disclosure and the contents of which are incorporated herein by reference. See also U.S. Pat. No. 6,215,120 to Gadeken et al., and U.S. Pat. No. 6,944,548 to Radtke et al.        
A drawback of these imaging methods is that while they reveal structural information about the earth formation, they provide little or no information about the formation mineral composition. The present disclosure addresses this deficiency and enables a more complete lithologic characterization of earth formations using MWD techniques.